Repeated evolution of unorthodox feeding styles drives a negative correlation between foot size and bill length in hummingbirds

Differences among hummingbird species in bill length and shape have rightly been viewed as adaptive, in relation to the morphology of the flowers they visit for nectar. In this study we examine functional variation in a behaviorally related but neglected feature: hummingbird feet. We gathered records of hummingbirds clinging by their feet to feed legitimately as pollinators or illegitimately as nectar-robbers—“unorthodox” feeding behaviors. We measured key features of bills and feet for 220 species of hummingbirds and compared the 66 known “clinger” species (covering virtually the entire scope of hummingbird body size) to the 144 presumed “non-clinger” species. Once the effects of phylogenetic signal, body size, and elevation above sea level are accounted for statistically, hummingbirds display a surprising, but functionally interpretable negative correlation. Clingers with short bills and long hallux (hind-toe) claws have evolved—independently—more than 20 times, and in every major clade. Their biomechanically enhanced feet allow them to save energy by clinging to feed legitimately on short-corolla flowers and by stealing nectar from long-corolla flowers. In contrast, long-billed species have shorter hallux claws, as plant species with long-corolla flowers enforce hovering to feed, simply by the way they present their flowers